let images = {};
let image_arr = {
  'bg': './images/background-1.jpg',
  'player': './images/我方飞机.png',
  'enemy': './images/ship_1.png',
  'friendly': './images/ship_2.png',
  'enemyBullet': './images/aestroid_dark.png',
  'meteorite': './images/aestroid_brown.png',
  'star5': './images/planets/001-global2.png',
  'star4': './images/planets/002-travel2.png',
  'star3': './images/planets/006-mars2.png',
  'star2': './images/planets/008-earth-globe2.png',
  'star1': './images/planets/012-jupiter2.png',
  'fuel': './images/燃料.png'
}
function $(dom) {
  return document.querySelector(dom)
}
function $all(dom) {
  return document.querySelectorAll(dom)
}